The study, published in the online medical journal PLoS Medicine, showed that on average, African doctors practised for 6.5 years before entering the US. The greatest increase in emigration among medical workers to the US was from Liberia, which in 2008 had only 1.37 doctors per 100,000 people compared with 250 per 100,000 in the US.

Only in South Africa was there a fall in departing doctors during the first decade of the century.

Akhenaten Benjamin Siankam Tankwanchi from the Peabody School of Education at Vanderbilt University in Nashville, Tennessee, and his fellow authors concluded that the research highlighted a “growing problem” and a “major loss” to Africa.

They said their calculations underestimated the true extent of “brain waste” because the US figures – drawn from the American Medical Association’s Physician Masterfile – showed only those African doctors who qualified for a medical residency and not others who had quit medicine to seek alternative employment.

The authors argued that emigration from Africa began in earnest in the mid-1980s following public sector budget cuts sought by international financial institutions, while immigration into the US was encouraged by relatively easy allocation of visas and permanent residency to qualified foreign physicians.

Even if destination countries somehow could somehow force the reversal of all skilled migration to date, this would do relatively little to address critical skill shortages broadly among origin countries. For example, all African-born doctors and nurses working in OECD countries in 2007 constituted only 12 percent of the World Health Organization's estimated shortage of health workers in the region. The same figure for Southeast Asia is 9 percent.

This is an important reason why a 2007 study of African health-care professionals found no significant effect of migration of nurses or doctors on a variety of health indicators including infant mortality, child mortality, vaccination rates, child respiratory infection and treatment, and HIV prevalence and treatment. The roots of skill shortages lie in complex problems of economic and institutional development that are largely beyond the reach of migration policy. Partly for this reason, the countries in Africa with the lowest rates of physician and nurse emigration are those with the worst overall health conditions--such as with the highest child mortality rates--not the best.

The financial effects of skilled migration, too, are often much smaller than conventionally believed. Researchers have estimated the total financial cost of physician emigration from sub-Saharan Africa, by summing the training costs of all Africa-trained doctors who have ever immigrated to the principal destination countries. Their highest estimate is that this cumulative sum would be offset by a one-time payment of US $2 billion. In the last five years alone, OECD countries gave $206 billion in development assistance to sub-Saharan African countries. That is, if any such debt is construed to exist, it is paid by three weeks of typical aid flows to sub-Saharan Africa. Moreover, these calculations overestimate the true financial cost of medical immigration. They ignore the direct and multiplier effects of remittances by emigrant physicians, as well as any service that these migrants provided at the origin before emigrating; while including the cost of their education before medical school--back to first grade.

The introduction of brain waste into the debate would seem to muddy the waters. An underestimate of immigrants would impact the assessments Clemens lists. However, it doesn't undermine his argument. He suggests we amass more information in order to better understand this talent flow, which the consideration of brain waste underscores. Finally, Clemens recommends planning for more geographic mobility. Restricting migration has many negative consequences. Another consideration is the welfare of the migrants themselves.

Michael Clemens is the inspiration for my tagline, "people develop, not places." The migrants benefit from the move from a developing country to a developed country. The debate about international migration usually concerns places. The populist rendition holds that sending and receiving countries pay dearly as if the migrants themselves, actual human beings instead of a patch of Earth, don't matter.

Talent retention is not a key driver of economic vitality. If it were, Chicago and New York City would be in a world of trouble. But that doesn't stop many from resorting to the tired brain drain refrain.